The present invention relates to methods and apparatus for separating contaminants from water, and more particularly relates, in one embodiment, to methods and apparatus for separating or dividing or removing oil, grease, solids and/or other suspended matter from produced water by a combination of flotation and centrifugation actions.
In many industries, including oil, paper and pulp, textile, electricity generating and food processing, there is an ever-present problem of contaminated water as a by-product of various processes. In particular, water is often associated with the production of oil and gas. Water subsequently produced from the formation (“produced water” originating from the operators or other sources) becomes contaminated with oil and solids encountered in the formation, and therefore cannot be disposed of simply by discharging into the surrounding water. Accordingly, numerous methods and systems have been devised to reduce the contaminant content of this produced water to a level allowed by local environmental laws and regulations. Treatment of produced water is particularly of interest in offshore oil and gas facilities, where majority of associated water is discharged overboard and into the open waters or reinjected into the reservoir or disposal wells. The ever tightening environmental regulations and associated cost with this additional treatment, is the driving force for new and improved methods of treatment.
U.S. Pat. No. 4,094,783 describes a multi-stage, recycling, centrifugal, flotation separator system that includes a circular cylindrical vessel with a vertical axis, having a horizontal tray positioned inside the vessel near the top, where the tray has an axial opening. Means are included for introducing air under pressure into the vessel at the top thereof, and other means are provided for introducing contaminated liquid into the vessel through a tangential pipe, under pressure, at a level below the tray. A mechanism is also provided to recycle air from the top of the vessel into the inlet line and to mix the recycled air with the contaminated liquid before entry into the vessel. There is additionally described means to recycle liquid from the bottom of said vessel, through a tangential pipe into the vessel at a level below that of the contaminated liquid, and means to mix recycled air with the recycled liquid prior to entry into the vessel. Although this system has a relatively small footprint for a flotation system because of its vertical orientation, it suffers from many of the same drawbacks with respect to the system disclosed in U.S. Pat. No. 4,255,262.
The overall construction and manner of operation of hydrocyclones are well known. A typical hydrocyclone includes an elongated body surrounding a tapered separation chamber of circular cross-section, the separation chamber decreasing in cross-sectional size from a large overflow and input end to a narrow underflow end. An overflow or reject outlet for the lighter fraction is provided at the wider end of the conical chamber while the heavier underflow or accept fraction of the suspension exits through an axially arranged underflow outlet at the opposite end of the conical chamber. (It will be appreciated that the terms “reject” and “accept” are relative and depend upon the nature and value of the lighter and the heavier fractions.) Liquids and suspended particles are introduced into the chamber via one or more tangentially directed inlets, which inlets create a fluid vortex in the separation chamber. The centrifugal forces created by this vortex throw denser fluids and particles in suspension outwardly toward the wall of the conical separation chamber, thus giving a concentration of denser fluids and particles adjacent thereto, while the less dense fluids are brought toward the center of the chamber and are carried along by an inwardly-located helical stream created by differential forces. The lighter fractions are thus carried outwardly through the overflow outlet. The heavier particles and/or fluids continue to spiral along the interior wall of the hydrocyclone and exit the hydrocyclone via the underflow outlet.
The fluid velocities within a hydrocyclone are high enough that the dynamic forces produced therein are sufficiently high to overcome the effect of any gravitational forces on the performance of the device. Hydrocyclones, especially those for petroleum fluid processing, are commonly arranged in large banks of several dozen or even several hundred hydrocyclones with suitable intake, overflow and underflow assemblies arranged for communication with the intake, overflow and underflow openings, respectively, of the hydrocyclones.
Hydrocyclones are used both for the separation of liquids from solids in a liquid/solid mixture (“liquid/solid hydrocyclones”) as well as for the separation of liquids from other liquids (“liquid/liquid hydrocyclones”). Different constructions are used for each of these hydrocyclone devices. Generally, the liquid/liquid type of hydrocyclone is longer in the axial direction than a solid/liquid hydrocyclone and is thinner as well. As a result of these structural differences, it cannot be assumed that the design and structure of a liquid/liquid hydrocyclone usefully translates to a liquid/solid hydrocyclone and vice versa.
In the recovery of hydrocarbons from subterranean formations, it is common that the fluids produced are mixtures of aqueous fluids, typically water, and non-aqueous fluids, typically crude oil. These fluid mixtures are often in the form of suspensions and/or dispersions together with solids or suspended matter that are all difficult to separate from the water. Hydrocyclones are known to be a useful physical method of separating oil phase fluids from aqueous phase fluids, along with other apparatus including, but not necessarily limited to, settling tanks, centrifuges, membranes, and the like. Additionally, electrostatic separators employ electrical fields and the differences in surface conductivity of the materials to be separated to aid in these separations.
As mentioned, “produced water” is the term used to refer to streams generated by the recovery of hydrocarbons from subterranean formations that are primarily water, but may contain significant amounts of non-aqueous contaminants dispersed therein. Typically, produced water results from an initial separation of oil and water, and accounts for a majority of the waste derived from the production of crude oil. After a primary process of separation from the oil, the produced water still contains drops or particles of oil in emulsion in concentrations as high as 2000 mg/l, and often solids or other suspended matter and thus it must be further treated before it may be properly discharged to the environment. Every country has set limits for the concentration of oil dispersed in the water for off-shore wells and for near-shore fields. Even if the produced water is returned to the field, it is advisable to remove as much of the oil and suspended solids (e.g., sand, rock fragments, and the like) as possible in order to minimize the risk of clogging the field.
Thus, conventional flotation units are bulky, heavy, and as noted, motion dependent or motion affected. There is a market demand for alternative technologies to reduce the footprint, weight and cost. It would also be helpful if a new technology did not depend upon a gas-liquid interface. It would be desirable if methods and apparatus were devised that could simultaneously remove oil and other non-aqueous species from produced water and contaminated water with greater efficiency than at present.